Data backup systems employing two computer systems are widely employed. Backup of data means that the data identical to those of one computer system are created and stored in another computer system. A computer system of a backup source is referred to as an active system and a computer system of a backup destination as a standby system. Further, data that are stored in the standby system are referred to as backup data. Such backup of data using a duplicating system is described in, for instance, CHRISTOS A. POLYZOIS, HECTOR GARCIA-MOLINA, “Evaluation of Remote Backup Algorithms for Transaction-Processing Systems”, ACM Transactions on Database Systems, Vol. 19, No. 3, September 1994, Pages 423–449 (hereinafter referred to as Reference 1).
A known method for backup using a duplicating system is a log transfer method. The log transfer method is a method in which data update records of an active system are transferred to a standby system whereupon backup data are created in the standby system. In the log transfer method, data of the active system and backup data of the standby system are made identical at the stage of starting backup. After start of backup, data update records of the active system are transferred to the standby system. In the standby system, backup data are updated using the transferred data update records. Results of studies in which the log transfer method is applied to a database system are described in Reference 1.
In general, computer systems have external storage devices, and data of computer systems are stored in the external storage devices. The external storage devices mean, for example, hard disk drives or disk arrays, which are magnetic storage devices.
Some of the external storage devices have copy functions for copying data between two devices. For instance, external storage device SANRISE9980V which is a product of Hitachi, Ltd. has HITACHI True Copy Function. The HITACHI True Copy Function is a function of transferring data via a network and of copying data of one SANRISE9980V to another SANRISE9980V. In case data are added to the external storage device of the copy source, the same data are added to the external storage device of the copy destination as well. When data of the external storage device of the copy source are overwritten, the data on the external storage device are also overwritten. Storage areas of the external storage devices are used upon dividing them into logical areas referred to as volumes, and whether copy functions are to be used or not can be set for each of the volumes.
Backup using the log transfer method is performed by using copy functions of external storage devices. The structure is as follows. At the stage of starting backup, data of the active system and backup data of the standby system are made identical. Data update records that have been added to the external storage device of the active system are copied to the external storage device of the standby system by using the copy functions to the volumes storing therein data update records. In the standby system, newest backup data are created using the copied data update records.